Liquid dispensing systems have become an integral part of the electronics manufacturing process for depositing underfill, encapsulants, solder fluxes, surface mount adhesives, conformal coatings and other materials onto a substrate, such as a printed circuit board. Each liquid dispensing system used in the electronics manufacturing process has a particular dispensing characteristic that is determined in large measure by the desired liquid dispense pattern on the substrate, the liquid flow rate and/or liquid viscosity of the dispensed material, and the desired electronic component assembly throughput through the dispensing system.
For example, in the assembly of ball gate arrays (BGA's) and other electronic components onto a ceramic or FR-4 substrate, the component must be soldered onto the substrate to form the necessary electrical interconnections. As each component occupies a predetermined area on the substrate, the liquid dispensing system must have the capability to dispense liquid or viscous material in a controlled manner within the selected component areas. Typically, the liquid dispenser is mounted on a movable platform to provide automated and accurate movement of the liquid dispenser in three dimensions relative to the substrate with the aid of a machine vision system.
Prior to the component soldering process for establishing the electrical interconnections, it is often necessary or at least desirable to dispense a layer of solder flux onto a substrate within rectangular areas associated with each component. To provide this capability, liquid material dispensers have been developed in the past that use filled syringes or reservoirs of solder flux, and dispensing valves to dispense droplets of flux material onto the substrate in a controlled manner with up to 25,000 to 40,000 dots of fluid per hour for a typical dispenser platform. These liquid dispensers, known as "dot jetting" dispensers, are programmed to dispense an array of liquid or viscous material droplets within each selected rectangular area which are then allowed to flow into contact with each other to form a generally rectangular thin layer of flux within the component area. However, notwithstanding the advances made in the electronics manufacturing process through "dot jet" dispensing of solder flux onto a substrate, the "dot jet" dispensing process has several drawbacks or limitations.
For example, in the "dot jet" process, each dispensed droplet has a generally circular configuration. To effectively cover or form a liquid layer in a rectangular area on the substrate, material flow of adjacent "dots" or "dot" overlap is required. These dispensing techniques can result in too much material being dispensed within the predetermined component area, or surface tension changes in the dispensed material that do not allow the "dots" to flow as desired. Moreover, due to the relatively small diameter of the "dots", more dispense cycles are required per unit area of coverage which can result in reduced throughput of electronic assemblies through the dispensing system.
Thus, there is a need for a liquid dispensing system and method that more effectively forms generally rectangular liquid layers on a substrate. There is also a need for a liquid dispensing system and method that improves throughput of assemblies through the dispensing system for forming rectangular liquid layers on a substrate.